1. Field of the Invention
The present invention relates to a laser driver and an optical disk drive apparatus provided with said laser driver.
2. Description of the Related Art
In a laser driver, it is desirable to ground either an anode or a cathode of the laser to suppress a surge current. Moreover, to drive the capacity-loaded laser, it is advisable to provide connection between a transistor, which is a switching element for turning on or off a current to the laser, and the laser as a collector follower. To satisfy such conditions, a conventional laser driver uses either a sink (inflow) current supply or a source (outflow) current supply.
FIG. 3 shows an example of a conventional laser driver in which only a sink current supply is used. Referring to the figure, one terminal of a laser 1 is grounded and the other terminal is connected to a collector of one transistor 3A of a differential current switching circuit 3, which comprises a pair of NPN transistors 3A and 3B. The collector of the other transistor 3B is grounded. To the bases of the transistors 3A and 3B, if the laser driver is used for an optical disk drive apparatus, write data signals are alternately applied inversely. The emitters of the transistors 3A and 3B, which are coupled to each other, are connected to a sink current supply 5. The sink current supply 5 is formed into a current mirror circuit and receives a constant (sink) current 12 in response to a (setting) current Il from a current setting circuit 7. In such a conventional construction, it is necessary to provide a negative source voltage -Vcc to the sink current supply 5. This involves a problem that the negative voltage source -Vcc must be provided in addition to a voltage source of the apparatus in which a circuit for driving the laser is used.
FIG. 4 shows an example of a conventional laser driver in which only a source current supply is used. Referring to the figure, one terminal of a laser 1 is grounded and the other terminal is connected to the collector of one transistor 13A of a differential current switching circuit 13, which comprises a pair of PNP transistors 13A and 13B. The collector of the other transistor 13B is grounded. To the bases of the transistors 13A and 13B, if the laser driver is used for an optical disk drive apparatus, write data signals are alternately applied inversely. The emitters of the transistors 13A and 13B, which are connected to each other, are connected to a source current supply 15. The source current supply 15 is formed into a current mirror circuit and provides a constant (source) current 14 in response to a setting current 13 from a current setting circuit 17. In such a conventional construction, a negative voltage source -Vcc is not needed, but PNP transistors must be used as switching elements. However, the PNP transistors, their switching operations being generally slow, causes a problem that it will be difficult to increase a switching speed of the laser.